1. Field of the Invention
This invention relates to a process for making 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetracyclo [5.5.0.0.5,9.03,11]-dodecane, also commonly known as 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane, HNIW or CL-20, and in particular relates to a continuous process for making HNIW.
2. Description of the Related Art
HNIW is a polycyclic caged nitramine oxidizer having the following chemical structure: 
For most existing weapons systems, the most critical ingredient in terms of propellant and explosive performance is the oxidizer. HNIW, with its substantial increase in performance output, presents significant opportunities in energy capabilities for propellant and explosive systems. It may be possible to replace existing weapons system energetic fillers with HNIW to increase shaped charge anti-armor penetration, increase missile payload velocity and standoff, increase underwater torpedo effectiveness and lethality, and improve gun propellant impetus.
In view of the potential plethora of applications for HNIW, it would be advantageous to develop a continuous process for making HNIW in high production capacities.
According to one known process for making HNIW, HNIW is synthesized via nitration of the precursor, tetraacetyldiformyl-hexaazaisowurtzitane (xe2x80x9cTADFxe2x80x9d), as shown below: 
TADF can be synthesized according to the procedure described in U.S. Pat. No. 5,739,325 to Wardle, entitled xe2x80x9cHydrogenolysis of 2,4,6,8,10,12-hexabenzyl-2,4,6,8,10,12-hexaazatetracyclo[5.5.0.0 5,9.03,11]dodecane,xe2x80x9d the complete disclosure of which is incorporated herein by reference. However, nitrolysis of TADF to form HNIW of acceptable purity has been found to require long reaction times, such as on the order of 2 to 3 hours. Attempts to increase the kinetics of the reaction and shorten reaction times have resulted in the formation of formyl-containing impurities. Thus, it would be extremely difficult to produce HNIW of acceptable purity via a continuous process which involves the nitrolysis of TADF to HNIW, since long residence times are required for the nitrolysis of the presursor, TADF, to HNIW.
A series of acyl group-containing hexaazaisowurtzitane derivatives is disclosed in EP 0 753 519 A1, the complete disclosure of which is incorporated herein by reference. Examples 20 and 22 of this European publication collectively disclose a process in which 2,6,8,12-tetraacetyl-2,4,6,8,10,12-hexaazatetracyclo[5.5.0.05,903,11]-dodecane (also known as tetraacetylhexaazaisowurtzitane or xe2x80x9cTADHxe2x80x9d) is, in a stepwise manner, first nitrosated with sodium nitrate and then oxidized with 100% nitric acid to form a dinitro intermediate, dinitrotetraacetylhexaazaisowurtzitane. After distilling off the nitric acid, the dinitro intermediate is reacted with a mixed acid consisting of 50 vol. % nitric acid (100%) and 50 vol. % sulfuric acid (100%) to form HNIW. However, it is reported that the second stage of the reaction took 8 hours at 0xc2x0 C. followed by 67 hours at room temperature to go to completion.
It would therefore be a significant advancement in the art to provide a reasonably rapid, continuous process for the formation of HNIW. In particular, it would be a noteworthy advance in the synthesis of HNIW to significantly shorten the nitrolysis of TADF or TADH to HNIW.
In accordance with the principles of this invention, the above-mentioned and other advances in the art are attained by the provision of a process in which TADF is replaced with 2,6,8,12-tetraacetyl-2,4,6,8,10,12-hexaazatetracyclo-[5.5.0.05,903,11]-dodecane (xe2x80x9cTADHxe2x80x9d), and subjecting the TADH to nitrolysis in the presence of a mixed acid to form HNIW. The mixed acid comprises at least one nitronium ion source and at least one strong acid preferably nitric acid and sulfuric acid, respectively) capable of generating nitronium ions from said source. The nitrolysis reaction is shown below: 
The volumetric ratio of nitronium ion source (e.g., HNO3) to strong acid (e.g., H2SO4) is preferably selected so that when reacted at a temperature of 85xc2x0 C., a product having undergone at least 99% conversion to nitramine is rapidly formed, preferably in no more than 10 minutes. More particularly, the volumetric ratio of HNO3:H2SO4 is preferably about 7:3.
As referred to herein, the purity of the HNIW product can be rated based on a xe2x80x9cconversion to nitraminexe2x80x9d standard. Conversion to nitramine means 100 multiplied by the ratio of the number of available substituted and unsubstituted nitrogen groups (of the analyzed nitramines) converted to nitramine groups (Nxe2x80x94NO2) divided by the total number of substituted (Nxe2x80x94R) and unsubstituted (Nxe2x80x94H) nitrogen groups (of the analyzed nitramines) that are capable of being converted to nitramine groups. The conversion to nitramine is determined by NMR analysis as follows. Analysis of HNIW by NMR produces two peaks. The first of these peaks represents the protons at the 3, 5,9, and 11-positions of HNIW, whereas the second of these peaks represents the protons at the 1-position and 7-position of HNIW. Generally, the first peak is approximately twice the area of the second peak, since the first peak accounts for twice as many protons as the second peak. Other nitramine impurities produced during HNIW synthesis are present during NMR analysis and produce their own distinct peaks. Nitramine conversion is determined by taking the ratio of the area of the second HNIW peak (for the 1,7-positioned protons) to the greatest area of any peak produced by protons of a nitramine other than HNIW, i.e., a nitramine impurity. Thus, a conversion to nitramine of at least 99% HNIW means that the smaller HNIW peak area of the 1,7-position protons from NMR analysis is at least 99 times the area of the peak of greatest area produced by protons of a nitramine impurity (that is, a nitramine other than HNIW).
In accordance with a preferred embodiment of this invention, the process of converting TADH to HNIW by nitrolysis with the mixed acid is conducted in a continuous manner.
These and other objects, features, and advantages of the present invention will become apparent from the following detailed description of the invention when taken in conjunction with the accompanying Figure which illustrates, by way of example, the principles of this invention.
The accompanying FIGURE is a schematic view of a flow diagram of a continuous process for the nitrolysis of TADH to HNIW in accordance with an embodiment of this invention.
As mentioned above, Examples 20 and 22 of EP 0 753 519 A1 collectively disclose a process in which TADH is selected as a precursor and, after being nitrosated and thereafter nitrated, is reacted with a mixed acid consisting of 50 vol. % nitric acid (100%) and 50 vol. % sulfuric acid (100%). However, it is reported that the mixed acid took 67 hours at room temperature to drive the reaction to completion.
Intuitively, and as dictated by known chemistry principles, it would seem that the rate of reaction would be increased by increasing the sulfuric acid concentration of the mixed acid, since an increase in the sulfuric acid concentration generates a corresponding increase in nitronium ion activity. However, the present inventors discovered, to their surprise, that high concentrations of strong acids, such as sulfuric acid, decrease the rate of HNIW formation. High proportions of such strong acids result in impure HNIW containing acetyl substituents being quickly precipitated from the reaction mixture before completion of the nitrolysis of TADH to HNIW. Hence, subsequent nitrolysis of the precipitated, partly nitrated TADH is consequently slowed.
The present inventors found that the reaction rate for the nitrolysis of TADH to HNIW can advantageously be increased by nitrolysis of the TADH with a mixture comprising nitric acid and a strong acid, preferably sulfuric acid, at prescribed volumetric ratios. The optimum volumetric ratio of nitronium ion source to strong acid is dependent upon the nitronium ion source and strong acid selected, as well as the amount of TADH present, since the concentration of TADH affects the amount of water generated and, hence, solubility. Generally, however, a volumetric ratio of HNO3:H2SO4 in a range of from about 6:4 to about 8:2 is preferred. At this preferred range, nitrolysis to 99% nitramine conversion can be achieved in less than 20 minutes at 85xc2x0 C. (A temperature of 85xc2x0 C. is presented for illustrative purposes only as a control temperature at which nitramine conversion is measured as a function of time, although 85xc2x0 C. is particularly suitable since it drives the kinetics of the reaction without causing the acids to boil. It is to be understood that this process may be carried out at other temperatures, but preferably is carried out to have ratios of mixed acid components and ingredients selected so that, if the treatment step had been carried out at 85xc2x0 C., a 99% conversion to nitramine would be obtained in not more than 10 minutes.) Most preferably, the volumetric ratio of HNO3:H2SO4 is about 7:3, so that the reaction is capable of proceeding at 85xc2x0 C. to form a product of 99% conversion to nitramine (NMR) in not more than 10 minutes. Another strong acid that can be used in lieu of sulfuric acid is methylsulfonic acid CH3SO2OH, although methylsulfonic acid should be used in a volumetric ratio closer to about 7:3 to provide the nitrolysis reaction with the capability of forming 99% conversion to nitramine at 85xc2x0 C. within 20 minutes.
The ratio of mixed acid (in milliliters) to TADH (in grams) is preferably at least about 7:1, can be in a range of from about 7:1 to about 30:1, more preferably is about 7.5:1 to about 10:1, and most preferably is about 8:1. At ratios less than about 7:1, the efficiency of the conversion to nitramine is adversely effected. Ratios greater than about 30:1 are discouraged for efficiency and economic reasons.
The acid mixture may contain up to about 5% by volume of water, but most preferably is substantially free of water, meaning that it has less than 2% by volume of water. More preferably, the acid mixture has less than 1% by volume of water.
The FIGURE depicts an exemplary continuous flow process for converting TADH to HNIW. In the illustrated process, TADH is fed through a screw feeder 10 into a first cooling vessel 12, which is preferably operated at about 0-20xc2x0 C. Also fed into the first cooling vessel 12 through conduit 14 containing a condenser 15 is a mixed acid, which is introduced in the prescribed concentrations to dissolve the TADH in about 2-3 minutes. The mixed acid preferably comprises a mixture of sulfuric acid and nitric acid, added in such amounts that the volumetric ratio of HNO3:H2SO4 is preferably about 7:3. A portion of the solution is continuously tapped off from the first cooling vessel 12 and fed through conduit 16 into a reactor 18, which contains condenser 19 and is operated at a temperature of about 85xc2x0 C. The solution is maintained in the reactor 18 for a residence time of 10 minutes to form HNIW, which is removed through conduit 20 and fed into a second cooling vessel 22. Ice is fed through feeder 24 into the second cooling vessel 22 to maintain the second cooling vessel 22 at an operating temperature of about 0-20xc2x0 C. The volumetric rate of the ice fed through feeder 24 is preferably equal to the volumetric rate of acid feed to the second cooling vessel 22. A product stream overflowing from the vessel 22 is filtered and collected.
TADH is available through Asahi of Osaka, Japan. As an alternative source of TADH, a process for preparing TADH will now be explained in detail. It is understood, however, that the invention is not to be restricted to this process embodiment.
TADH can be prepared from hexabenzylhexaazaisowurtzitane (HBIW) as a precursor. HBIW can be synthesized according to the procedure described by Nielsen et al. in xe2x80x9cPolyazapolycyclics by Condensation of Aldehydes with Amines. 2. Formation of 2,4,6,8,10,12-Hexabenzyl-2,4,6,8,10,12-hexaazatetracyclo[5.5.0.05,9.03,11 ]dodecanes from Glyoxal and Benzylamines,xe2x80x9d Journal of Organic Chemistry, Vol. 55, pp. 1459-66 (1990) and U.S. Pat. No. 5,723,604, the complete disclosures of which are incorporated herein by reference. It is understood, however, that equivalents of HBIW might also be used, such as precursor compounds containing substitutions for one or more of the benzyl groups.
HBIW is subjected to a hydrogenolysis step to form tetraacetyldibenzylhexaazaisowurtzitane (xe2x80x9cTADBxe2x80x9d), as described in U.S. Pat. No. 5,739,325 and as shown below: 
In this preparatory process, a quantity of HBIW, a cosolvent, and a bromine source are placed into a reaction vessel. Representative of suitable cosolvents are N,N-dimethylformamide (xe2x80x9cDMFxe2x80x9d), N-methylpyrollidone (xe2x80x9cNMPxe2x80x9d), and 1,2-dimethoxyethane. Suitable bromine sources include molecules having active bromine, such as benzyl bromide, acetyl bromide, and bromine gas (Br2). The HBIW, cosolvent, and bromine source are preferably mixed in an atmosphere which is substantially non-reactive with hydrogen. For instance, the reaction vessel can be purged with an inert atmosphere, such as nitrogen. Alternatively, the reaction vessel atmosphere can be removed by vacuum. Acetic anhydride and a palladium hydrogenolysis catalyst are added to the reaction vessel, followed immediately by introduction of hydrogen into the reaction vessel.
The hydrogenolysis catalyst is preferably added to the reaction vessel in an amount less than 10% by weight based on the HBIW substrate. Typical hydrogenolysis catalysts which can be used include Pd(OH)2, Pd, and mixtures thereof on carbon commonly used as a catalyst support. Several standard palladium metal and Pearlman""s-type catalysts have both been found to be suitable. Such catalysts are commercially available. Similarly, catalysts that are provided either water-wet or dry have been useful. The weight percent of active palladium on carbon is preferably less than 10%, more preferably less than 5%, and can be as low as 3%.
The hydrogenolysis catalyst is preferably substantially free of water. This can be accomplished by washing the catalyst with the co-solvent prior to introduction into the reaction vessel to remove water associated with the catalyst. The palladium catalyst is normally shipped water-wet, with approximately 50% of the weight being water. While not wishing to be bound by theory, it is presently believed that acetic acid, formed by reaction of acetic anhydride in the reaction mixture with the water on the catalyst, reduces the yield and increases the chances of a failed or incomplete reaction. Previous efforts at water removal, such as vacuum drying, which was unacceptable due to fire hazard and catalyst activity reduction, or washing with acetic anhydride, did not fully remove water and left acetic acid present. Washing with the polar co-solvent effectively removes water and does not introduce deleterious side products or reduce catalyst activity.
It has been discovered that addition of the reactive acetic anhydride immediately prior to hydrogen introduction improves the reaction yield, rate, and reproducibility. The acetic anhydride is added immediately prior to hydrogen introduction so that the acetic anhydride does not have time to react with the HBIW to form N-benzylacetamide which acts as a catalyst poison, which is a major contributor to incomplete or low yield reactions. N-benzylacetamide is formed by the acid catalyzed decomposition of HBIW to yield xe2x80x9cfreexe2x80x9d benzyl amine followed by acetylation of the benzyl amine by acetic anhydride. This reaction occurs slowly once the reaction mixture is together. To minimize this unwanted reaction, the co-solvent and HBIW are preferably placed in the reaction vessel first, followed by the bromine source. The contents are thoroughly mixed and placed under a nitrogen atmosphere. The acetic anhydride and the washed palladium catalyst are then added quickly, followed immediately by hydrogen introduction. Once the acetic anhydride is added to the HBIW, the hydrogen should be added rapidly to inhibit unwanted side reactions.
Once hydrogen is introduced into the reaction vessel, the HBIW is converted to TADB, which precipitates onto the palladium hydrogenolysis catalyst and is easily recovered by filtration. The co-solvent assists in providing complete precipitation. After the hydrogenolysis is complete, the product and catalyst are filtered from the liquid phase and washed with a solvent, such as denatured ethanol, methanol, or isopropanol. The solvent is preferably miscible with the DMF, acetic anhydride, and acetic acid so that these compounds can later be removed from the TADB product. The solvent is also preferably immiscible with the desired TADB product so that the TADB is not dissolved and washed away with the solvent. It is also important that the solvent have not effect on the subsequently hydrogenolysis reaction.
The filtered and washed TADB is sufficiently pure for a second hydrogenolysis reaction in which the TADB product and catalyst are reacted to form TADH, as depicted as follows: 
A suitable catalyst and solvent for the second hydrogenolysis reaction are Pd(OAc)2 and acetic acid, respectively. The second hydrogenolysis reaction is slower than the first hydrogenolysis reaction, due to the reduced activity of the last two benzyl groups of the TADB towards hydrogenation. This second hydrogenolysis step is preferably performed in the absence of formic acid or other acid to prevent the formation of TADF or equivalent compounds.
The catalyst is removed by filtration, and the product is recovered by evaporation of the volatile solvents. The catalyst may be recycled and used again in the process or it may be reprocessed by the catalyst manufacturer. The TADH product thus obtained is of a purity suitable for direct use in the nitration reaction to produce HNIW.
The HNIW can be thereafter crystallized to xcex5-polymorph HNIW in accordance with the technique disclosed in allowed U.S. Pat. No. 5,874,574 the complete disclosures of which are incorporated herein by reference. (According to this U.S. patent, a quantity of CL-20 is dissolved in a solution containing a CL-20 solvent, such as ethyl acetate, and water. The resulting mixture consists of two liquid phases: an aqueous phase and a wet solvent phase. The pH of the aqueous phase can be tested and adjusted at this point as desired. The CL-20 is dissolved in the wet solvent phase, which is dried by removing a solvent/water azeotrope according to conventional distillation techniques, thereby forming a dry solvent phase containing the CL-20. A low density non-polar CL-20 non-solvent, such as hexane, cyclohexane, heptane, octane, benzene, toluene, xylene, mineral oil, petroleum ethers, and ligroin, is added to the dry CL-20 solvent phase to cause crystallization of xcex5-polymorph CL-20. The low density non-polar non-solvent preferably has a density less than water. The CL-20 crystals are then separated from the non-solvent and the solvent by adding sufficient water to displace the non-solvent and the solvent from the surface of the xcex5CL-20 crystals. In this fashion, xcex5-polymorph CL-20 is made wet for later handling, packaging, and shipping. The ratio of water to CL-20 should typically range from 1:7 to 3:1, by volume. More water can be easily used in the system, but larger quantities of water will require larger equipment for separating and recycling the water. The wet CL-20 is collected and the CL-20 non-solvent, CL-20 solvent, and excess water are removed to separate and recycle the individual solvents.)
The conversion to nitramine as reported and claimed herein is determined by NMR standards by the procedure detailed above in the Summary and the Example below. However, it is to be understood that other standards for determining nitramine conversion exist, although such other standards may alter the appropriate timing for obtaining 99% conversion and, therefore, should not be used in determining the literal and equivalent scope of the appended claims. One alternative, unclassified standard for determining HNIW purity was developed pursuant to a Standardization Agreement between the North Atlantic Treaty Organization (NATO) and the Military Agency for Standardization (MAS). This standard is STANAG4566. Under this Standardization Agreement, chemical purity is determined by using high performance liquid chromatograph (HPLC) equipped with a 226 nm UV detector and an integrator or computer link up to a data acquisition. Generally, a conversion of 97% (as measured by the STANAG4566) can be achieved in less than 30 minutes, preferably in less than 20 minutes.
The unexpected superior results obtained by practicing the present invention are manifested by the following experiments, which are presented for the purpose of explanation and are not to be considered limiting on the scope of the invention.